It has long been recognized in the temperature control art that the level of humidity can be correlated with the degree of comfort at any particular temperature. In the control of indoor temperatures, it has been common to add humidity to the air during the winter heating season, and to try to remove humidity during the summer cooling season. Despite the fact that the interrelationship between humidity and temperature is well recognized, it has not been a common practice to try to control humidity directly with temperature to maintain a particular level of comfort.
By and large, most thermostats are electromechanical or mechanical in structure, and it has been very difficult to measure humidity. There has been some attempt to mechanically interlink a humidity control with a temperature control, but this type of device is rather expensive, complex, and is not susceptible of retrofit once a thermostat has been manufactured and/or installed. For the most part, humidity has been controlled totally independent of the temperature control and usually is set at some range dependent on outdoor temperatures. Typically, if humidity control is added to a thermostatically controlled system, it is as a modification of the output of the thermostat control system.